Certain superimposed steering systems are conventional and an aspect of which is that, if required, a further rotational angle can be superimposed by an actuator on the steering angle which is selected on a steering handle by a driver of a vehicle. The additional rotational angle is controlled by an electronic control and/or regulating device and serves to increase the driving stability of the vehicle or other purposes.
In order to produce an additional angle or a further rotational angle, it is described in German Published Patent Application No. 101 29 450 and German Published Patent Application No. 101 60 313 to use a planetary gear mechanism as variable ratio gear unit, a steering shaft which is connected to a first gear unit input shaft being disconnected axially and being connected rotatably by a multistep planet which moves around sun gears of the steering shaft parts. The multistep planets are mounted in a planetary gear carrier. The planetary gear carrier can in turn be moved about the steering shaft and its sun gears with a helical gear mechanism or a worm gear mechanism, the rotational movement of the planetary gear carrier being carried out with the aid of a helical gear or worm on a second gear unit input shaft of the variable ratio gear unit.
Variable ratio gear units of this type have a large number of toothing engagements. This results in gear unit play which can produce undesirable steering play. In addition, the abovedescribed variable ratio gear units with servomotor are relatively complicated and expensive in technical terms.
German Published Patent Application No. 102 20 123 describes a superimposed steering system, an electric servomotor drive being integrated into a steering column shaft which connects a steering wheel to a steering gear. The electric servomotor drive has a housing which is connected to a first section of the steering column shaft and a drive shaft which is connected to a second section of the steering column shaft. The first section of the steering column shaft is connected to a steering wheel and the second section is connected to the steering gear, or vice versa. The drive shaft can be rotated by the electric servomotor drive relative to the housing of the electric servomotor drive, in order to produce an additional rotational angle. Although an additional angle is superimposed with this superimposed steering system on a rotational movement between the steering wheel and the steering gear, the opposing torque of the electric servomotor drive is supported on the steering column shaft, which can lead to an uncomfortable steering or driving sensation for a driver.
Superimposed steering systems are conventional which have a harmonic drive or pulsator drive as variable ratio gear unit. However, the servomotor of harmonic drives of this type supports its torque on the steering column. In addition, these harmonic drives require a stable housing in order to transmit the torque and the rotational angle at the steering handle to the gear unit output shaft of the variable ratio gear unit.